Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a radiation-sensitive material (such as, for example, photoresist) associated with a semiconductor wafer. The radiation-patterning tool can be referred to as a photomask or a reticle. The term “photomask” traditionally is understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized interchangeably to refer to radiation-patterning tools that define patterns across some or all of a wafer.
Reticles contain light restrictive regions (for example, totally opaque or attenuated/half toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel spaced conductive lines on a semiconductor wafer.
Photolithography initially comprises forming a layer of radiation-sensitive material (such as, for example, photosensitive resist material, which is commonly referred to as photoresist) over a wafer. Subsequently, radiation is passed through the reticle onto the layer of photoresist, and a pattern defined by the reticle is transferred onto the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
An example prior art photolithography process is described with reference to FIGS. 1-3.
FIG. 1 shows a reticle 10 comprising a base (or body) 12. A series of radiation-altering structures 11, 13, 15 and 17 are provided over a surface of the body, and a series of regions (specifically gaps) 14, 16 and 18 are between the radiation-altering structures. The radiation-altering structures 11, 13, 15 and 17, together with the gaps 14, 16 and 18 define a pattern imparted to radiation passing through reticle 10. Although the pattern imparted to the radiation is defined by the structures and gaps, the pattern may differ from the specific pattern of the structures and gaps due to interference effects occurring in the radiation as it passes through the reticle. Such interference effects are accounted for in the design of the reticle.
The body 12 may comprise, consist essentially of, or consist of material transparent, or at least substantially transparent, to radiation passed through the reticle during photolithography; and may, for example, comprise, consist essentially of, or consist of quartz. The radiation-altering structures 11, 13, 15 and 17 may comprise materials which block a significant percentage of electromagnetic radiation from passing therethrough (for instance, chromium-containing materials), and/or may comprise materials which shift the phase of electromagnetic radiation passing therethrough (for instance, molybdenum silicide). The radiation-altering structures may comprise materials formed across a surface of base 12 (as shown) or may comprise patterns etched into base 12 (for instance, may comprise grating patterns etched into base 12 to shift a phase of electromagnetic radiation passing through the base).
The structures 11, 13, 15 and 17 may define relatively opaque portions of the reticle, and the gaps 14, 16 and 18 may define relatively transparent portions of the reticle. The terms “relatively opaque” and “relatively transparent” are utilized to indicate regions which are more opaque or transparent relative to one another, respectively, and may include, but are not limited to, regions which are completely opaque or completely transparent, respectively. Together, the relatively opaque regions and relatively transparent regions pattern radiation passing through the reticle.
The regions 14, 16 and 18 are shown to consist of unmodified locations of base 12. In some application the regions are modified prior to photolithography by, for example, forming materials within the regions, and/or by recessing at least portions of the regions into the base.
The base 12 has a pair of opposing sides 7 and 9. The pattern of structures 11, 13, 15 and 17, and regions 14, 16 and 18, is formed along side 7. The patterned side 7 may be referred to as a front side of the reticle, and the opposing side 9 may be referred to as a backside of the reticle.
FIG. 2 shows an apparatus 20 utilizing reticle 10 for patterning radiation. Apparatus 20 contains a semiconductor substrate 22 having a radiation-sensitive material 26 over a base 24. The base may comprise, consist essentially of, or consist of monocrystalline silicon. The radiation-sensitive material may comprise positive or negative photoresist. The radiation-sensitive material may be particularly sensitive to one or more wavelengths of radiation, such as, for example, wavelengths shorter than 300 nanometers.
The terms “semiconductive substrate,” “semiconductor construction” and “semiconductor substrate” mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. Although base 24 is shown to be homogenous, the base may comprise numerous layers in some applications. For instance, base 24 may contain one or more layers associated with integrated circuit fabrication. In such applications, such layers may correspond to one or more of metal interconnect layers, barrier layers, diffusion layers, insulator layers, etc.
The semiconductor substrate 22 is provided beneath the front side 7 of reticle 10. Electromagnetic radiation 28 is patterned by passing it through reticle 10. The radiation may comprise a plurality of wavelengths, with one or more of the wavelengths being the predominate wavelengths utilized for patterning the radiation-sensitive material. The wavelengths predominately utilized for patterning the radiation-sensitive material may be referred to as the “primary” wavelengths utilized in the printing of a pattern into the radiation-sensitive material.
The patterned radiation impacts radiation-sensitive material 26 to print a pattern within the radiation-sensitive material. The printed pattern comprises exposed regions 30 and non-exposed regions 32. The exposed and non-exposed regions are shown separated by dashed-line boundaries to assist in illustrating the exposed and non-exposed regions.
FIG. 3 shows semiconductor substrate 22 after development of the radiation-sensitive material 26 to remove the exposed regions 30 selectively relative to the unexposed regions (alternatively, the development may remove the unexposed regions selectively relative to the exposed regions). The development forms a plurality of openings 34, 36 and 38 extending through the radiation-sensitive material to the underlying base 24. In subsequent processing, an etch may be conducted to extend the openings into the base.
In many semiconductor fabrication processes, it is desired to form a large array of identical openings extending through a radiation-sensitive material. For instance, is often desired for memory to comprise large arrays of identical structures. FIG. 3 shows a desired result in which the openings are uniformly created to the same dimensions as one another (in other words, in which the openings have uniform critical dimensions across base 24). In practice, such desired result is often not achieved.
FIG. 4 illustrates an example problem that may occur during formation of the openings 34, 36 and 38. Specifically, FIG. 4 shows that opening 36 has not entirely penetrated through the radiation-sensitive material 26. Such may be due to a problem with the substrate 24 (for instance, the substrate may not have a planar topography of the upper surface), a problem with the radiation-sensitive material (for instance, the radiation-sensitive material may not have been formed to uniform thickness across the substrate), or a problem during the exposure of FIG. 2 (for instance, one of the openings in the pattern on the reticle may not have had appropriate dimension relative to the other openings and/or the intensity of electromagnetic radiation through part of the reticle may not have been the same as the intensity through another part of reticle).
FIG. 5 illustrates another example problem that may occur during formation of openings 34, 36 and 38. Specifically, the openings 34, 36 and 38 are not uniform in critical dimension; with openings 34 and 38 being shown to be wider than opening 36. Such problem may result from, for example, non-uniformity in dimensions of the openings in the pattern on the reticle.
The same reticle and processing methods may be utilized for sequential processing of numerous substrates. A problem that occurs on one substrate will frequently occur on all of the sister substrates processed with the same reticle and processing conditions. Accordingly, it is desired to develop procedures for curing photolithographic problems before they are propagated to a large number of substrates.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and a decrease in the dimensions of conductor elements which connect those integrated circuit devices. The demand for ever smaller integrated circuit devices brings with it demands for ever decreasing dimensions of structural elements, and ever increasing requirements for precision and accuracy in radiation patterning.
Control of critical dimension uniformity during photolithographic formation of openings may be of increasing importance as ever higher levels of integration are sought for integrated circuit fabrication. Efforts have been made to improve critical dimension uniformity by improving the reticles utilized for photolithography. One method is to darken a reticle by using a laser to damage quartz. The damaged quartz attenuates light passing through the reticle, which may induce birefringence, and which may thereby affect critical dimension by reducing the degree of polarized light (in other words, by shifting a phase of the polarized light). This may be undesirable in some applications.